Recording head for magneto-optical recording device and magneto-optical recording device comprising the same

ABSTRACT

A recording head for magneto-optical recording in a first-surface recording configuration comprises a transparent aperture ( 8;22;32 ), in a plane intended to be substantially parallel to a recordable medium ( 2 ), in use. The head is arranged to provide an optical path allowing a bundle of light ( 6 ) to pass through the transparent aperture ( 8;22;32 ) onto the recordable medium ( 2 ), and further comprises a central magnetic head structure ( 11;20;34 ) comprising a flux guide ( 12;24;35 ), positioned so as to partially obstruct the transparent aperture ( 8;22;32 ). The central magnetic head structure ( 11;20;34 ) is of a substantially larger dimension in a first direction (x) parallel to the plane than in a second direction (y) parallel to the plane.

The invention relates to a recording head for magneto-optical recordingin a first-surface recording configuration, comprising

a transparent aperture, in a plane intended to be substantially parallelto a recordable medium, in use,

the head being arranged to provide an optical path allowing a bundle oflight to pass through the transparent aperture onto the recordablemedium, and further comprising

a central magnetic head structure comprising a flux guide, positioned soas to partially obstruct the transparent aperture.

The invention further relates to a magneto-optical recording devicecomprising such a recording head.

An example of a recording head and device of the types mentioned aboveis known from WO 00/31734. The known head is part of a system comprisinga magnetic head, an optical sub-system with a radiation source forgenerating a beam of radiation, a recordable medium suitable forrecording or erasing information therein or thereon under combinedcontrol of the magnetic head and radiation source. Head and source arepositioned on the same side of the recordable medium. The system furthercomprises a reflective medium to reflect the radiation incident on itfrom the source onto the recordable medium. The magnetic head is locatedin the optical path between radiation source and recordable medium so asto obstruct the beam only partly. The reflective medium serves to createa focal point of the optical system at a location in the recordablemedium.

A problem of the known configuration is that the recording layer issituated in between the recording head and the reflective medium and thelight has to pass through the layer during recording. As this layer isgenerally metallic, its transmission coefficient is low. This isespecially problematic when the recorded data is also read out by meansof a light beam passing through the recording head, as is the case formagneto-optical recording using the Kerr effect, for example. Thereflective medium is necessary in order to apply sufficient light energyto a spot in the recording layer of the recordable medium.

It is an object of the invention to provide a recording head andrecording device of the types mentioned above, which provide both amagnetic field and a light beam of sufficient strength and intensity,respectively, on the recordable medium, while enabling the magneticfield and light beam to be generated with relatively little power.

This object is achieved by the recording head according to theinvention, which is characterized in that the central magnetic headstructure is of a substantially larger dimension in a first directionparallel to the plane than in a second direction parallel to the plane.

The invention is based on the recognition that, in order to obtain arelatively high magnetic field strength for a given field-generatingcurrent, the central magnetic head structure, and in particular the fluxguide part, should have a large cross-sectional area. On the other hand,obscuring part of the spot generated by the beam of light will lead to asharpening of the spot profile and the occurrence of side lobes. Sidelobe intensities can be kept small when the obscuration is small, sothat it is advantageous to keep them small in one direction to preventthermal heating by the side lobes and thus broadening of the thermalprofile, which could otherwise corrupt adjacent bits recorded on themedium.

In a preferred embodiment, the central magnetic head structure comprisesat least one electrically conductive winding.

Thus the magnetic field used for recording is at least partly generatedby passing a current through these windings. Because they are in thecentral magnetic head structure, they can have a small diameter, thusresulting in a coil with a relatively low self-inductance. This has theadvantage of making the recording head more suitable for high bit-raterecording and generally all recording techniques requiring frequentreversal of the direction of the magnetic field, for examplelaser-pulsed magnetic field modulation.

In a preferred embodiment, the flux guide comprises a central polehaving an axis substantially centred on the bundle of light passingthrough the transparent aperture.

Thus, the magnetic flux is concentrated in a small area around the spotin the recordable medium on which the bundle of light is focussed. Thisallows a highly efficient magnetic head, which can be operated from alow power driver.

In an embodiment combining the two previously described embodiments, atleast one winding is wound around the central pole.

Thus, the highest possible magnetic head efficiency at the lowestself-inductance is achieved, since the magnetic field is generated onlyin a small area around the optical spot and the compact size of themagnetic head and its windings lead to a relatively low self-inductance.

Preferably, the recording head comprises at least one return pole,having an axis parallel to the axis of the central pole.

Thus, the flux in the recordable medium is concentrated, since the fluxlines will be concentrated on a path from the central pole to the returnpole, rather than spreading out through the recordable medium. Thisincreases the effectiveness of the recording head.

An advantageous embodiment further comprises an annular outer magnetichead structure, surrounding the central magnetic head structure andhaving an inner diameter exceeding the diameter of a bundle of lightpassing through the aperture.

The annular outer magnetic head structure preferably comprises a coilfor enhancing the magnetic field created by the central magnetic headstructure, but may comprise only a yoke for concentrating the fluxgenerated by the latter. Thus, the magnetic field strength created isenhanced. The annular magnetic head structure does not obstruct thebundle of light, so its dimensions are less critical. Without theannular outer magnetic head structure, the magnetic field strength wouldbe constrained by the dimensions of the central magnetic head structure,which may not obscure the beam of light too much. Furthermore, due toits compactness, the central magnetic head structure comprisingelectrically conductive windings may, when used on its own, heat up, soreducing the efficiency. Because at least part of the magnetic field isgenerated by the central magnetic head structure, with its windings ofsmall diameter, the recording head allows faster switching than arecording head comprising only the annular outer magnetic headstructure.

Preferably, the recording head comprises a catadioptric optical system,arranged to provide the optical path.

This has the advantage that, in a split-optics configuration, in whichthe optics for reading data are separated from those for recording, theoptical spot and magnetic recording field are kept well aligned uponsmall variations in the angle of the bundle of light entering theoptical system. Furthermore, the recording head remains relativelycompact. In particular, the mass of the recording head can thus be keptlow. This is of importance for both recording heads comprising a sliderand those that are actuated in order to keep the beam of light focussedto a spot in the recording layer of the recordable medium.

Preferably, the catadioptric optical system is arranged to transform anincoming bundle of light into a hollow bundle of light.

In this respect, a hollow beam of light is to be taken to mean a beam oflight of which the intensity profile taken over the cross-sectional areais much lower in the center than elsewhere. This embodiment has theadvantage that the central magnetic head structure can be placed in thecenter of the hollow beam of light, thus avoiding loss of light powerthrough absorption in and reflection off the central magnetic headstructure.

In a further embodiment, the central magnetic head structure extendsbeyond an outer dimension of the aperture in the first direction.

Thus, it is possible to channel externally generated magnetic flux intothe central magnetic head structure.

According to another aspect of the invention, the magneto-opticalrecording device according to the invention comprises a recording headaccording to the invention.

The invention will now be explained, by way of example, in furtherdetail with reference to the accompanying drawings, in which:

FIG. 1 shows schematically, and not to scale, a cross-section of a firstconfiguration for a recording head;

FIG. 2 is a bottom view of the aperture of the recording head of FIG. 1;

FIG. 3 is a schematic perspective view of the magnetic head structure ofFIG. 1;

FIG. 4 shows schematically, and not to scale, a cross-section of arecording head with a catadioptric focussing system and a central aswell as an outer magnetic head structure;

FIG. 5 is a bottom view of the aperture of the recording head of FIG. 4;

FIG. 6 is a schematic perspective view of the central and outer magnetichead structures of FIG. 4;

FIG. 7 shows schematically, and not to scale, a cross-section of asecond type of recording head with a catadioptric focussing system,comprising merged central and outer magnetic head structures;

FIG. 8 is a bottom view of the aperture of the recording head of FIG. 7;and

FIG. 9 is a schematic perspective view of the merged central and outermagnetic head structures in the recording head of FIG. 7.

A recording head 1, as shown in FIG. 1, is provided for magneto-opticalrecording of information onto a recordable medium 2. The recordablemedium 2 is shown only very schematically, with only a substrate 3, athin-film recording stack 4 and a cover layer 5 being indicated. It isto be understood that a real recordable medium will comprise morelayers, in particular the thin-film recording stack 4 will comprise manymore layers, differing in composition. Information is recorded byfocussing a bundle of light 6 onto a spot in the thin-film recordingstack 4 and applying a magnetic field in a specific direction, inaccordance with the information to be written. Because the light isincident on the thin-film recording stack 4 without passing thesubstrate 3, this recording configuration is referred to asfirst-surface recording. Simply put, except for the (optional) thincover layer 5, the layer comprising the recording stack 4 is the firstlayer encountered by the incident light.

The recording head 1 comprises a glass substrate 7, with an aperture 8for allowing light to pass onto the recordable medium 2. The outerdiameter of the aperture 8 is larger than the outer diameter of thebundle of light passing through it, so as not to diminish the amount oflight transmitted. The outer diameter of the bundle of light 6 thatpasses through the aperture 8 is determined by the diameter of anincoming bundle of light 9 and the optical system comprised in therecording head 1 for transforming the incoming bundle of light 9 intothe bundle of light 6 focussed on a recording spot in the recordablemedium 2. Thus, the recording head 1 is arranged to provide an opticalpath, allowing a bundle of light 6 to pass through the aperture 8. Inthe example of FIG. 1, the optical system is formed principally by alens 10.

In the configuration of FIG. 1, the magnetic field is generated by meansof a central magnetic head structure 11. The inner part of the bundle oflight 6 is obscured by the central magnetic head structure 11.

It will be apparent from FIG. 2 that the inner aperture 8 has an outerdiameter large enough to allow the hollow bundle of light 6, formed fromthe incoming bundle of light 9 to pass through substantiallyunobstructed at its perimeter. By contrast, the central magnetic headstructure 11 is positioned in the optical path along which the hollowbundle of light 6 is transmitted, so as to partially obstruct thetransparent aperture 8 from the center outwards.

It can be seen that the central magnetic head structure 11 is of asubstantially larger dimension (i.e. an order of magnitude higher thanthe usual manufacturing tolerances) in a first direction, marked as thex-direction in FIGS. 2 and 3, than in a second direction perpendicularthereto, marked as the y-direction in FIGS. 2 and 3. The centralmagnetic head structure 11 is thus rotationally asymmetric. Note that inthe reference system of FIGS. 2 and 3, the x- and y-axis aresubstantially parallel to the plane in which the aperture 8 lies, whichin turn is substantially parallel to the recordable medium 2, in use. Itis further observed that the cross-section perpendicular to thedirection in which light is emitted from the recording head 1 can be anyother rotationally asymmetric shape, i.e. elliptical, star-shaped,crescent-shaped, etc. The rectangular shape is just an example.

FIG. 3 shows an advantageous embodiment of the central magnetic headstructure 11. It comprises an E-shaped yoke 12 of soft magneticmaterial, acting as a flux guide. The yoke 12 is processed usinghard-disk like thin-film technologies. Its processing plane is inprinciple perpendicular to the y-direction, although it may also beperpendicular to the z-direction. Suitable materials include NiFe andCoZrNb. The yoke 12 comprises a central pole 13, aligned along thez-axis. In use, the central pole 13 is substantially centered on thebundle of light 6 passing through the aperture 8 (cf FIG. 2). Theoptical system is arranged to focus the bundle of light 6 onto a spotlying substantially on the continuation of a longitudinal axis of thecentral pole 13. Thus, the magnetic field is concentrated on the spot onwhich the bundle of light 6 is focussed.

A number of turns 14 of electrically conductive material form a windingaround the central pole 13. The winding is preferably also formed usingthin-film technology. Although the winding may be made of metal, in anadvantageous embodiment, at least some of the turns 14 are made of atransparent material, such as Indium Tin Oxide (ITO). In particular,inner turns immediately adjacent the central pole 13 may be of metal,whereas outer turns at a distance from the central pole are made of ITO.Such a combination has the advantage that the outer turns do notobstruct the bundle of light, thereby increasing the amount of lightthat can be focussed on the recording spot, whereas the inner turns havea lower resistance, thus increasing the field generation efficiency anddecreasing the amount of heat dissipated in the winding.

The other legs of the E-shaped yoke 12 are formed by return poles 15,16,which serve to obtain a good magnetic efficiency. It is observed thatthis depends partially on the distance between the central pole 13 andthe outer return poles 15,16. The E-shaped yoke 12 allows an optimumdistance to be determined using known methods of magnetic fieldcalculation in a relatively straightforward manner.

It is observed that the partial obscuration of the bundle of light nearthe aperture leads to the sharpening of the light intensity profile inthis direction (i.e. the x-direction in the example). However,side-lobes also arise. This effect is less marked in the otherdirection, i.e. the y-direction in the example. It is possible toarrange the recording device comprising the recording head 1 in such amanner that the recording tracks are in the y-direction. Alternatively,they may be in the x-direction.

Whichever configuration is chosen, it is important that the recordingmagnetic field area on the disk stays aligned with the spot on which thebundle of light 6 is focussed. A way of doing this in a split opticsconfiguration, i.e. where the laser and detection optics (not shown) aremechanically separated from the recording head 1, is to use a light pathwhere the bundle of light is fixed with respect to the focussing lens10. An example is a sliding head with integrated lens and a fibre fixedto the slider for coupling the light into the lens. The preferred way,shown in FIGS. 4 and 7 in two exemplary embodiments, is by means of acatadioptric optical system. A catadioptric system has the advantagethat, when the stop is on the exit surface of the lens, the magneticfield and focussed spot stay in better alignment upon variations in theangle of the incoming bundle of light 9 than in a conventional focussingsystem, because of the small free working distance (distance to therecordable medium 2). Furthermore, catadioptric lenses have theadvantage of being relatively compact. In addition, they allow, in theshown embodiments, a relatively high numerical aperture, as well as theaforementioned relatively small free working distance. The relativelyhigh numerical aperture allows more light to be focussed on therecording spot for a given cross-sectional area of the aperture 8. Thesmall free working distance allows stronger magnetic fields to beachieved at the area of recording on the recordable medium 2 for a givencurrent through the winding(s).

In FIG. 4, the optical system comprises a catadioptric focussing lens17. The optical system transforms the incoming bundle of light 9 intothe hollow bundle of light 6. This is due to the fact that the focussinglens comprises a convex surface part 18, on a side opposite the side ofthe focussing lens 17 at which the incoming bundle of light 9 enters thefocussing lens 17. The convex surface part 18 is covered by a reflectivecoating, indicated by means of a dotted line. An annular part 19 of theopposite surface of the focussing lens 17 is also covered by a coatingthat is reflective at the wavelengths present in the incoming bundle oflight 9. Thus, light is reflected off the convex surface part 18, ontothe annular part 19 and then passes through the remaining surface partadjacent the aperture 22, to be focussed on the recording spot. Theedges of the incoming and transformed bundle of light 9,6 are shown, inorder to demonstrate that the incoming bundle of light 9 is transformedinto the hollow bundle of light 6, which substantially circumvents acentral magnetic head structure 20.

In the configuration of FIG. 4, the magnetic field is generated by meansof a central magnetic head structure 20, of which the operation isenhanced by means of an annular outer magnetic head structure 21. Theannular outer magnetic head structure 21 serves to increase the magneticflux generated by the central head structure 20 at the recording spot onthe recordable medium 2.

It will be apparent from FIG. 5 that the inner diameter of the outermagnetic head structure 21 is large enough to allow the hollow bundle oflight 6, formed from the incoming bundle of light 9, to pass throughsubstantially unobstructed at its perimeter. In contrast, the centralmagnetic head structure 20 is positioned in the optical path along whichthe hollow bundle of light 6 is transmitted, so as to partially obstructa transparent aperture 22 in a glass substrate 23 similar to those ofthe embodiment shown in FIG. 1. In fact, in the example of FIG. 4, someof the hollow bundle of light 6 actually impinges on the centralmagnetic head structure 20. In general, however, this need not be thecase. That is to say the hollow bundle of light could have an innerdiameter large enough to prevent any light from falling onto the centralmagnetic head structure 20.

FIG. 6 shows an advantageous embodiment of the central magnetic headstructure 20. It comprises a T-shaped yoke 24 made of soft magneticmaterial, acting as a flux guide. In use, a central pole 25 issubstantially centered on the (hollow) bundle of light 6 passing throughthe aperture (cf. FIG. 4). The optical system is arranged to focus thebundle of light 6 on a spot lying substantially on the continuation of alongitudinal axis of the central pole 25. Thus, the magnetic field isconcentrated on the spot on which the bundle of light 6 is focused.

A number of turns 26 of electrically conductive material form a windingaround the central pole 25. The T-shaped yoke 24 is combined with anannular outer magnetic head structure 21, comprising a plurality ofturns 27 of a circular winding. By applying a current through thesewindings, the field in the center can be further enhanced.

In the embodiment of FIG. 7, another type of catadioptric focussingsystem is used. The embodiment of FIG. 7 also comprises a focussing lens28. A central area 29 of the entrance surface is concave. A peripheralarea 30 is convex and coated with a reflective coating. A central part31 of the exit surface adjacent to an aperture 32 in a substrate isco-planar with the aperture 32 and has a high transmittance. Aperipheral part 33 of the exit surface is coated with a reflectivecoating. The concave central area 29 spreads the incoming beam of light,of which a substantial part is reflected by the peripheral part 33 ofthe exit surface and subsequently by the peripheral area 30 of theentrance surface, thus creating a hollow beam focussed through thecentral part 31 of the exit surface and the aperture 32.

In FIG. 8, an alternative central magnetic head structure 34 is shownthat fully obscures the outgoing bundle of light 6 in the x-direction.

FIG. 9 shows the central magnetic head structure 34 in more detail. Itcomprises a T-shaped yoke 35 of soft magnetic material, acting as a fluxguide. In use, a central pole 36 is substantially centered on the(hollow) bundle of light 6 passing through the aperture 32 (cf. FIG. 7).The optical system is arranged to focus the bundle of light 6 on a spotlying substantially on the continuation of a longitudinal axis of thecentral pole 36. Thus, the magnetic field is concentrated on the spot onwhich the bundle of light 6 is focussed.

A number of turns of electrically conductive material form the windingsof two coils 37,38, one around each outer leg of the T-shaped yoke 35.Because the windings are situated outside the bundle of light 6, thedimensions of the coils 37,38 and the number of turns are not limited byoptical requirements. Flux generated by the outer coils is transportedthrough the T-shaped flux guide to the central pole 36 and concentratedon the recording area.

The catadioptric embodiments have the advantage that they are relativelyeasy to manufacture within narrow tolerance ranges. This is due to theuse of coated focussing lenses 17,28 of which the surface can bepolished to the required shape.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage. For instance, the annular outer magnetichead structure may provide the return poles for completing a magneticcircuit, which also includes the central pole of the central magnetichead structure.

1. Recording head for magneto-optical recording in a first-surfacerecording configuration, comprising a transparent aperture (8;22;32), ina plane intended to be substantially parallel to a recordable medium(2), in use, the head being arranged to provide an optical path allowinga bundle of light (6) to pass through the transparent aperture (8;22;32)onto the recordable medium (2), and further comprising a centralmagnetic head structure (11;20;34) comprising a flux guide (12;24;35),positioned so as to partially obstruct the transparent aperture(8;22;32), wherein the central magnetic head structure (11;20;34) is ofa substantially larger dimension in a first direction (x) parallel tothe plane than in a second direction (y) parallel to the plane. 2.Recording head according to claim 1, wherein the central magnetic headstructure (11;20;34) comprises at least one electrically conductivewinding.
 3. Recording head according to claim 2, wherein at least someturns (14) of a winding are made of a transparent material.
 4. Recordinghead according to claim 1, wherein the flux guide (12;24;35) comprises acentral pole (13;25;36) having an axis substantially centered on thebundle of light (6) passing through the transparent aperture (8;22;32).5. Recording head according to claim 2, wherein at least one winding iswound around the central pole (13).
 6. Recording head according to claim4, comprising at least one return pole (15,16), having an axis parallelto the axis of the central pole (13).
 7. Recording head according toclaim 1, wherein the flux guide (12) comprises an E-shaped yoke. 8.Recording head according to claim 1, further comprising an annular outermagnetic head structure (21), surrounding the central magnetic headstructure (20) and having an inner diameter exceeding the diameter of abundle of light (6) passing through the aperture (22).
 9. Recording headaccording to claim 1, comprising a catadioptric optical system, arrangedto provide the optical path.
 10. Recording head according to claim 9,wherein the catadioptric optical system is arranged to transform anincoming bundle of light (9) into a hollow bundle of light (6). 11.Recording head according to claim 1, wherein the central magnetic headstructure (34) extends beyond an outer dimension of the aperture (32) inthe first direction (x).
 12. Recording head according to claim 2,wherein at least one winding (37;38) is wound around a part of the fluxguide (35) positioned outside a perimeter of the aperture (32). 13.Magneto-optical recording device, comprising the recording headaccording to claim 1.